Length measuring systems of this kind are already known from Published, Non-Examined German Patent Applications DE-OS 17 73 460 and DE-OS 27 30 715. In these length measuring systems, two scanners spaced apart in the measuring direction are provided for scanning the incremental graduation of the portions of the scale. In DE-OS 17 73 460, in addition to the incremental graduation, switch cams or switch slots are provided, which turn the scanners on in alternation. The location of the switch cams or switch slots is chosen such that before a point where two scale portions abut one another is reached, the scanner that was active until then was shut off, while at the same time the other scanner, already located on the adjacent portion of the scale, is turned on. Expensive adjustment operations at the scale abutment points can thus be dispensed with, since by the alternating activation of the two scanners, the portions of the scale at the abutting points are not scanned.
In DE-OS 27 30 715, which is a point of departure for the present invention, a control track is mounted on each portion of the scale, next to the incremental graduation. The control track for each scale portion is black, or in other words is not permeable to light, beginning at one end of each portion, and ends with a transparent window in a different end region.
These known length measuring systems have the disadvantage that over the entire measurement length, the measurement is done only incrementally; it is not possible to determine the absolute position.
To overcome this disadvantage, in the length measuring system of DE-OS 39 28 592, the switchover position is determined by an external detector located outside the length measuring system. This detector detects the absolute positions over the entire measuring region. However, this measuring system has the disadvantage of requiring not only the incremental length measuring system but also a further, absolute measuring system.